Differential diagnosis in ptosis and ophthalmoplegia: mitochondrial disease or myasthenia?

Mitochondrial Mutations Can Alter Neuromuscular Transmission in Congenital Myasthenic Syndrome and Mitochondrial Disease

Congenital myasthenic syndromes (CMS) are a group of rare, neuromuscular disorders that usually present in childhood or infancy. While the phenotypic presentation of these disorders is diverse, the unifying feature is a pathomechanism that disrupts neuromuscular transmission. Recently, two mitochondrial genes-SLC25A1 and TEFM-have been reported in patients with suspected CMS, prompting a discussion about the role of mitochondria at the neuromuscular junction (NMJ). Mitochondrial disease and CMS can present with similar symptoms, and potentially one in four patients with mitochondrial myopathy exhibit NMJ defects. This review highlights research indicating the prominent roles of mitochondria at both the pre- and postsynapse, demonstrating the potential for mitochondrial involvement in neuromuscular transmission defects. We propose the establishment of a novel subcategorization for CMS-mitochondrial CMS, due to unifying clinical features and the potential for mitochondrial defects to impede transmission at the pre- and postsynapse. Finally, we highlight the potential of targeting the neuromuscular transmission in mitochondrial disease to improve patient outcomes.

A Heterozygous Mutation in MFF Associated with a Mild Mitochondrial Phenotype

BACKGROUND

The number of mutations in nuclear encoded genes causing mitochondrial disease is ever increasing. Identification of these mutations is particularly important in the diagnosis of neuromuscular disorders as their presentation may mimic other acquired disorders.We present a novel heterozygous variant in mitochondrial fission factor (MFF) which mimics myasthenia gravis.

OBJECTIVE

To determine if the MFF c.937G>A, p.E313K variant causes a mild mitochondrial phenotype.

METHODS

We used whole exome sequencing (WES) to identify a novel heterozygous variant in MFF in a patient with ptosis, fatigue and muscle weakness. Using patient derived fibroblasts, we performed assays to evaluate mitochondrial and peroxisome dynamics.

RESULTS

We show that fibroblasts derived from this patient are defective in mitochondrial fission, despite normal recruitment of Drp1 to the mitochondria.

CONCLUSIONS

The MFF c.937G>A, p.E313K variant leads to a mild mitochondrial phenotype and is associated with defective mitochondrial fission in patient-derived fibroblasts.

Motor end-plate analysis to diagnose immune-mediated myasthenia gravis in seronegative patients

This study aimed to evaluate the diagnostic usefulness of motor end-plate (MEP) analysis along with clustered acetylcholine receptor (AChR) antibody (Ab) assays in patients with myasthenia-like symptoms but negative routine AChR and muscle-specific kinase (MuSK) Ab tests. MEP analysis of muscle biopsies of the biceps brachii was performed in 20 patients to try to differentiate between those with or without immune-mediated myasthenia gravis (MG). Using a quantitative method, complement C3 deposition and AChR densities in MEPs were examined. Independently, cell-based assays were used to detect serum clustered-AChR Abs. Only five of 20 patients had complement deposition at MEPs; four of these patients had reduced AChR densities similar to those in patients with typical AChR Ab positive MG, and distinct from those in the remaining 15 patients. Two of the four serum samples from these patients had clustered-AChR Abs. All complement-positive patients were considered as having immune-mediated MG and improved with appropriate treatments; although one patient presented with MG 3 years later, the remaining patients had other diagnoses during over 10 years of follow-up. These results suggest the usefulness of MEP analysis of muscle biopsies in diagnosing immune-mediated MG in seronegative patients with myasthenia-like symptoms but, due to the invasiveness of the muscle biopsy procedure, clustered AChR Abs should, if possible, be tested first.

Early-onset ophthalmoplegia, cervical dyskinesia, and lower extremity weakness due to partial deletion of chromosome 16: A case report

BACKGROUND

We explored the genotype-phenotype correlation of the novel deletion 16p13.2p12.3 in an 8-year-old child with progressive total ophthalmoplegia, cervical dyskinesia, and lower limb weakness by comparing the patient’s clinical features with previously reported data on adjacent copy number variation (CNV) regions.

CASE SUMMARY

Specifically, we first performed whole-exome sequencing, CNV-sequencing, and mitochondrial genome sequencing on the patient and his parents, then applied “MitoExome” (the entire mitochondrial genome and exons of nuclear genes encoding the mitochondrial proteome) analysis to screen for genetic mitochondrial diseases. We identified a de novo 7.23 Mb deletion, covering 16p13.2p12.3, by both whole-exome sequencing and CNV sequencing. We also detected 16p13.11 in the deleted region, which is the recurrent distinct region associated with neurodevelopmental disorder. However, the patient only displayed features of progressive total ophthalmoplegia, cervical dyskinesia, and weakness in his lower limbs without neurodevelopmental disorder. The “MitoExome” sequencing was negative. Brain magnetic resonance imaging revealed non-specific sporadic changes in the occipital parietal lobe and basal ganglia.

CONCLUSION

Taken together, these results indicated that 16p13.2p12.3 deletion causes a syndrome with the phenotype of early-onset total ophthalmoplegia. The “MitoExome” analysis is powerful for the differential diagnosis of mitochondrial diseases. We report a novel copy number variant in this case, but further confirmation is required.

Neuromuscular Junction Abnormalities in Mitochondrial Disease: An Observational Cohort Study

Objective

To determine the prevalence of neuromuscular junction (NMJ) abnormalities in patients with mitochondrial disease.

Methods

Eighty patients with genetically proven mitochondrial disease were recruited from a national center for mitochondrial disease in the United Kingdom. Participants underwent detailed clinical and neurophysiologic testing including single-fiber electromyography.

Results

The overall prevalence of neuromuscular transmission defects was 25.6%. The highest prevalence was in patients with pathogenic dominant RRM2B variants (50%), but abnormalities were found in a wide range of mitochondrial genotypes. The presence of NMJ abnormalities was strongly associated with coexistent myopathy, but not with neuropathy. Furthermore, 15% of patients with NMJ abnormality had no evidence of either myopathy or neuropathy.

Conclusions

NMJ transmission defects are common in mitochondrial disease. In some patients, NMJ dysfunction occurs in the absence of obvious pre- or post-synaptic pathology, suggesting that the NMJ may be specifically affected.